This invention relates to a process and apparatus for the activated sludge purification of sewage by aeration of a sewage-activated sludge mixture with oxygen or oxygen-enriched air.
In the activated sludge process for oxidizing sewage, the sewage is brought into contact, in suitable tanks, with aerobic living bacteria which can be suspended in the water in the form of sludge flakes, together with the simultaneous introduction of oxygen. During this step, a portion of the impurities to be broken down is converted into a bacterial substrate and another portion is reacted with oxygen to form essentially CO.sub.2 and water.
In most cases, the process is conducted by feeding air to an open activation tank by means of suitable aeration devices and mixing the air with the contents of the tank, so that the oxygen contained in the air is available to the bacteria in the aeration tank for respiration purposes.
One disadvantage of this process lies in poor utilization of the energy required during oxygen introduction, since a large portion of the oxygen and the entire nitrogen (78% by volume) of the air leaves the aeration tank after being used merely for intermixing the contents of the tank.
Due to economic considerations, it is impractical in this process to supply the tank with pure oxygen or with oxygen-enriched air, since here again the largest portion of the oxygen, obtained under energy expenditure, will escape unused. This is true even though pure oxygen or oxygen-enriched gas exhibits the advantage of providing a greater supply of oxygen to the tank and/or an increase in the oxygen concentration and thus an intensification of the purifying effect of the bacterial sludge.
In one attempt to overcome these difficulties, a "bio-precipitation" process has been described in Sewage Works Journal, September 1949, wherein the purification system consists essentially of two tanks. Sewage is enriched in oxygen in the first tank and then brought into contact with bacterial sludge in the second tank. This process has the disadvantage of separating the point where oxygen is introduced from the situs of oxygen consumption, whereby the concentration gradient of the oxygen, the most important factor of oxygen introduction, is diminished and consequently the oxygen activity is reduced. Furthermore, due to the limited solubility of oxygen, it is also necessary to provide an uneconomical water circulation cycle in order to meet the oxygen demand.
Another conventional process described in U.S. Pat. No. 3,547,811 utilizes closed aeration tanks which are subdivided into chambers. In this procedure, the sewage-activated sludge mixture flows successively through the several chambers where it is treated with a gas. Pure oxygen is fed to the first chamber, the exiting gas being partially recirculated in this chamber and partially conveyed to the second chamber, where it is again partially recirculated, and so forth. Finally, the gaseous mixture leaving the last chamber, insofar as it is not again recirculated therein, is exhausted to the atmosphere. This process has the disadvantage that the oxygen passing through the chambers is increasingly enriched in gaseous metabolic products, particularly CO.sub.2, so that a concentration gradient occurs which is reduced in oxygen from stage to stage, resulting in reduced oxygen activity. Furthermore, this process entails incomplete oxygen exploitation, since marked amounts of oxygen must necessarily be discharged from the last stage. The sewage is also enriched with metabolic products, particularly CO.sub.2, in the same manner, so that the bacterial sludge conveyed in parallel flow with the sewage is constantly exposed to a varying environment which results in reduced metabolic bacterial activity and thus reduced sewage purification.
A further disadvantage of this process is the relatively large capital and operating expenditures required for recirculating the activating gas in each one of the individual chambers.